This invention relates to digital systems in which multiple transmitters and multiple receivers are interconnected to each other on a transmission line; and more particularly, it relates to circuits for reducing ringing in the signals that are sent over such a transmission line.
In many digital systems of the prior art, several functional units are interconnected to each other over a data bus. This bus includes a plurality of transmission lines which carry respective digital signals in parallel. Each transmission line of the bus goes to all of the units in the digital system, and all the units provide a respective transmitter or receiver, or both, for each transmission line.
With such a system, it is desirable to be able to transmit data over the bus from one unit to another at a high rate. However, each of the transmission lines which make up the bus will inherently have a parasitic inductance, resistance, and capacitance; and that will limit the speed at which transmissions on the bus can occur. In particular, the parasitic inductance and capacitance of a transmission line can cause the signals on the line to ring when they switch from a one voltage state (e.g.--low) to another (e.g.--high). Such ringing must be allowed to settle before the data on the transmission line can be received; and that adds to the total transmission time.
In the prior art, this problem is addressed by U.S. Pat. No. 5,023,488 issued Jun. 11, 1991 to Gunning. FIG. 1 of this patent shows a digital system in which a transmission line has multiple transmitters and receivers connected to it; FIG. 4 shows a circuit diagram for each transmitter on the transmission line; and the FIG. 4 transmitter is structured to compensate for ringing. This is pointed out by Gunning at column 2, lines 48-59.
However, one problem with the Gunning transmitter is that in order to reduce ringing on the transmission line, the rise time of the transmitter output signal is increased (i.e.--the slope of the signal is decreased). This does reduce ringing on the transmission line; however, as the transmitter output signal travels down the transmission line, the signal's rise time is decreased even further by the parasitic capacitance and resistance of the line. Thus, the time for the signal to reach a reference level at which it is interpreted as a digital high, gets larger and larger. Further, as the signal rise time increases, the time delay through a receiver which detects when the signal is at the reference level, increases.
Also, another drawback of the Gunning transmitter is that the compensation which the transmitter provides is only applied by one transmitter at a time on the transmission line. That is, only one transmitter at a time can send signals on the transmission line, and the Gunning transmitter can compensate for ringing only if it is doing the sending. However, a ringing signal sent by one transmitter will propagate along the entire length of the transmission line to all of the other transmitters and receivers, and there delay the signals reception.
Accordingly, a primary object of the invention is to provide an electronic control module at each transmitter on a transmission time, which reduces ringing in a signal from one transmitter as the signal passes by every transmitter, and which does so without slowing down the switching speed of any transmitter.